The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the production of perfusion images in a fast cardiac gated MRI acquisition.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or "longitudinal magnetization", M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, improves image quality by reducing motion artifacts, and enables the performance of medical test procedures such as timed pharmacological stress tests (e.g. multi-stage dobutamine stress test). There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. When applied to cardiac imaging, for example, a complete scan from which a series of images showing the heart at different phases of its cycle or at different slice locations can be acquired in a single breath-hold.
Faster scan times can be achieved by segmenting k-space and acquiring multiple phase encoding k-space views per cardiac cycle. The scan time is speeded up by a factor equal to that of the number of k-space views acquired per image per R-R interval. In this manner, a typical CINE acquisition with a matrix size of 128 pixels in the phase encoding direction can be completed in as little as 16 heart beats, when 8 k-space views per segment are acquired.
Multiple slice locations can be visualized by repeated acquisition of the same k-space segment within each R--R interval, but by exciting different slice locations each at a different phase of the cardiac cycle. The number (n) of slice locations (S.sub.1 -S.sub.n) during a cardiac cycle is determined by the time needed to acquire data for a single segment and the length of the cardiac R--R interval: EQU N=R--R time/vps.times.TR
where vps is the number of k-space views per segment, and TR is the pulse sequence repetition time. The total scan time is then ##EQU1## where yres is the total number of phase encoding views in the image. Typically, an image utilizes 128 or more phase encoding views, and 8 views per segment is also often used.
In segmented k-space scans, the total scan time can be substantially reduced by increasing the number of views per segment (vps). However, as indicated by the above, this is at the expense of reducing the number of slice locations that can be acquired during each cardiac cycle.
Imaging of blood perfusion in tissue is closely related to the imaging of blood flow in vascular structures. Angiography, or the imaging of vascular structures, is very useful in diagnostic and therapeutic medical procedures. As with MR angiography, MR perfusion imaging is typically performed by injecting a bolus of an MR active contrast agent into the patient during an imaging session. These agents can either decrease the T1 of blood to enhance the detected MR signal (e.g. Gd-DTPA), or they can decrease the T2 of blood to attenuate the detected MR signal (e.g. iron oxide particles). As the bolus passes through the body, the enhanced (or attenuated) signal created by the bolus increases (or decreases) the signal intensity observed in perfused tissue, but not in non-perfused tissue. The degree of signal change in the observed tissue can be used to determine the degree of tissue perfusion.
Since perfusion measurements are based on the strength of the NMR signals acquired during the scan, it is very important that the NMR signal strength be made insensitive to other measurement variables. One such variable is the magnitude of the longitudinal magnetization M.sub.z which is tipped into the transverse plane by the rf excitation pulse in the NMR pulse sequence. After each such excitation, the longitudinal magnetization is reduced and then it recovers magnitude at a rate determined by the T.sub.1 constant of the particular spins being imaged. If another pulse sequence is performed before the longitudinal magnetization has recovered, the magnitude of the acquired NMR signal will be less than the signal produced by a pulse sequence which is delayed long enough to allow full recovery of the longitudinal magnetization. It is important in perfusion imaging that this variable (i.e. longitudinal magnetization) be maintained at a constant level throughout the scan.
When cardiac gating is used to control the acquisition of NMR perfusion image data, the time interval between acquisitions can vary considerably with a consequent variation in the longitudinal magnetization. This is particularly true if the subject has an irregular heart beat (i.e. arrhythmia). One solution to this problem is to apply an rf saturation pulse to the subject just prior to each pulse sequence, or pulse sequence segment (for each slice) and allow a fixed recovery time (TI) to occur before performing the pulse sequence. Unfortunately, unless the recovery time TI is fairly lengthy, the resulting NMR signals will be very small with a consequent reduction in the acquired NMR signals and the picture SNR. On the other hand, lengthening the recovery period TI lengthens the time (T.sub.seg) required to perform each pulse sequence segment EQU T.sub.seg =TI+vps.times.TR,
and reduces the number of slice locations that can be acquired during a cardiac R--R interval. In other words, there is a direct trade-off between image quality and the number of locations that can be acquired in a single breath-hold scan.